Conventional can-annular industrial gas turbine engines have a rotor shaft that spans the compressor section, the combustion section, and the turbine section, and which is constructed as a single piece. High pressures and temperatures contained by the industrial gas turbine engine outer casing provide motivation to keep the outer casing as small as possible. This leads to outer casing designs that follow the shape of the internal components of the engine. The overall shape of the industrial gas turbine engine, and the fact that the outer casing mimics the overall shape, make it impossible to create a single casing that encloses the entire engine. Consequently, the outer casing is usually an assembly of different casing sections assembled about the engine internal components.
In an industrial gas turbine engine where a single piece rotor shaft spans different engine sections, the casing sections are usually split into an upper half and a lower half to facilitate assembly and disassembly of the engine. Leaving the casing bottom halves assembled while removing the top halves also enables access to interior portions of the engine while providing a structural backbone that holds components of the engine in place during maintenance, such as when only certain internal components may be removed and replaced. As a result, conventional industrial gas turbine engines typically have an upper and lower casing that may roughly correspond to a compressor section of the engine, an upper and lower casing that may roughly correspond to a combustion section, and an upper and lower casing that may roughly correspond to a turbine section.
This configuration yields a horizontal joint where the upper and lower casings meet that runs along each side of the industrial gas turbine engine. Further, a circumferential joint is formed around the engine where axially adjacent casings abut. All joints present an opportunity for leakage leading to less efficient engine performance. Furthermore, casings are thicker where there are joints and thinner where there are no joints, leading to the potential for differential thermal expansion. To mitigate the effect of differential thermal expansion, longer startup and shut down times may be used. Further, differential thermal expansion during any transient temperature changes may cause an ovaling of the casing. This ovaling may be detrimental to internal components which count on a circular shape for the casing for proper performance, such as to maintain a desired blade clearance or for proper seal performance. Further, where the horizontal joint and a circumferential joint meet, a four way joint is formed. Four way joints are particularly challenging with respect to mechanical design considerations.
Current industrial gas turbine engine technology provides a maximum pressure ratio of about 22:1. That is, the compressor compresses air to a maximum of approximately 22 times the pressure of ambient air before the air is delivered to the combustors. The mechanical compression alone increases the temperature of the compressed air to approximately 440° C. Conventional split casings made of steel and within the combustion section, where the highest pressures and temperatures occur, may be near their maximum mechanical capacity when at 22 atmospheres and approximately 440° C. However, industrial gas turbine engines operate more efficiently with greater pressure ratios. Thus, conventional industrial gas turbine engine casing designs may inhibit the progress of industrial gas turbine engine development